1. Field of the Invention
The present invention pertains generally to encoding of information signals and, in particular, to digital encoding of video signals using differential pulse code modulation (DPCM).
2. Description of the Prior Art
In recent years, great strides have been made in reducing the redundancy in digitally encoded video signals, thereby allowing transmission of pictures over facilities having reduced bandwidth without unduly sacrificing fidelity. Recent improvements have taken many directions. For example, certain interframe coding systems take note of the fact that relatively few changes occur in corresponding picture elements in successive frames; accordingly, such systems detect these changes, and transmit information concerning only active areas. However, interframe systems requires considerable memory capability, and can thus be costly to implement.
Other prior art video encoding systems, particularly those of the differential pulse code modulation (DPCM) type, place great emphasis on the logic circuits used to generate a prediction of the next input sample. The prediction is subtracted from the input signal to yield an error signal, which, once quantized, becomes the next output signal. At the receiver, the same prediction logic is used to reconstruct the original input signal. In such systems, the predictions are obtained from various combinations of previous encoder outputs. Since better predictions can be obtained using more samples, the combinatorial logic can become quite complex and costly, and the same logic must be used twice, in both the transmitter and in the receiver.
Yet other prior art video encoders achieve increased encoding efficiency by using various compression/expansion characteristics in the quantizer, and by otherwise adapting the quantizer characteristic in accordance with one or more attributes of the input signal. For example, companding may be linear, exponential, logarithmic, or some combination of these or other functions. Further, quantizers having a coarse characteristic may be used where the picture area is busy or active, while a fine characteristic (using more bandwidth) is reserved for relatively flat or inactive picture regions. These techniques, while generally successful, again have certain limitations. For example, in the situation where "future" information is used to control the quantization characteristics, the availability of multiple quantizers in the encoder requires transmission to the receiver of information identifying which one was utilized. This requirement adds to the number of bits needed to represent a given picture element.
In view of the foregoing, it is the broad object of the present invention to reduce the redundancy in an encoded version of a video signal with apparatus that is not unduly complex or costly. Specific objects are to provide an intraframe encoder (not requiring a large frame memory) which has a generally simple predictor disposed in its feedback loop, and which eliminates the need to transmit to the receiver information concerning the particular quantization characteristic used in the encoding process, even though "future" information is used in switching the quantization characteristics.